Photodimerization of coumarin revisited: Effects of solvent polarity on the triplet reactivity and product pattern

The photoprocesses of parent coumarin (1) and a series of 6-alkyl substituted coumarins (2–11, alkyl chain lengths between 1 and 16 C-atoms) in solution were studied by time-resolved UV-vis absorption spectroscopy. The triplet properties under direct and benzophenone-sensitized excitation conditions are outlined, whereas any singlet state reactions towards photodimers are unlikely. The quantum yield of intersystem crossing of 1–11 is small, Φisc = 0.02–0.05, in benzene, acetonitrile or methanol and much larger, Φisc = 0.1–0.5, in water and 2,2,2-trifluoroethanol. The rate constant for intrinsic decay was found to be below 1 × 105 s−1 in most cases. The observed triplet state reacts with coumarins; the rate constant for this self-quenching step is largest in aqueous solution, 3 × 109 M−1 s−1, and smallest, 3 × 107 M−1 s−1, in benzene. The results for the derivatives are very similar and the main difference is a lower Φisc for 1 in aqueous solution. The published product yields and patterns from preparative irradiations as a function of solvent polarity are discussed in view of the triplet properties.